For present seismic streamer positioning applications, depth, heading, and hence position of an individual section of seismic streamer cable is determined and controlled either through the utilization of internally-mounted sensors or externally-mounted pods which are clamped onto the seismic streamer at various locations, with the electronics being hard wired or in the case of pods being battery operated and inductively coupled through the streamer casing to cables within the streamer. Examples of the pod type systems include U.S. Pat. Nos. 3,931,608; 3,611,975; 3,704,681; 3,673,556; 3,648,642; and 3,531,761.
The mounting of sensing devices wholly within the streamer has the advantage that the electronics are contained wholely within the streamer free from damage. Examples of these types of systems include those described in U.S. Pat. Nos. 4,160,229 and 3,909,774. However, one particularly severe drawback of internally housing the electronics is that for calibration purposes, it may be necessary to remove a particular streamer section from the array. This means that both electrical and mechanical connections must be severed between the streamer sections, the section opened up, and the unit calibrated. Thus, locating compasses or depth sensors within the streamer has a decided disadvantage that if a unit is malfunctioning, shorted or broken, it is extremely hard to remove it from the streamer. Moreover, as discussed in U.S. Pat. No. 4,160,229, sensors are, in essence, clamped within the cable to two or more strain members. This produces differential strains which cause canting of the sensor housing due to slightly different lengths of strain members. This causes errors because the sensor is not aligned with the longitudinal axis of the streamer.
It will be appreciated that in the usual seismic streamer application, it is usual to electrically wire compasses or heading sensor behind each other either in a daisy chain series connection, behind each other in parallel, or individual wires are run to each unit. In the usual application there are, for instance, ten heading sensing sections in a three kilometer streamer, with one heading sensor every 300 meters. Assuming that a streamer section is 50 meters long, one compass is provided for every six sections.
By way of further background, it should be noted that in most streamer applications, many compasses are used to determine the positions of the streamer segments. One type system utilizing a plurality of magnetic compasses is described in U.S. Pat. No. 4,231,111.
With respect to calibration of the compasses carried internal to the streamer, it is a common practice to string streamers with internal compasses behind the boat with, for instance, ten compass sections in a row, and the remaining portion of the streamer, including the acoustic sections, streaming out beyond the compass sections. The ship then tows the streamer on several headings and recordings are made of the outputs of the various heading sensors or compasses, with heading sensors compensated by comparing their outputs. Here, the assumption is that the compasses are all in a straight line and should have the same reading as an output. If the readings are different, this difference is stored in the processing equipment carried aboard the ship and the output of the particular unit is corrected accordingly. This, of course, is a time consuming process.
After calibration, the units are dispersed down the length of the streamer which is again a relatively time consuming process. Moreover, the distribution of the heading sensors down the streamer asumes that variations have not taken place during remounting.
It will be appreciated that clamped-on pods have a calibration advantage in that instead of taking apart the sections, the pods are moved to the front of the array and behind the ship for calibration. However, the use of externally-mounted pods has some very severe problems. First, the pod has to be removed if the cable is reeled in. Since this is accomplished often under not the best of sea states, it is an essentially risky job and external pods are often dropped overboard. The second problem is that the electronics within the pod are battery-operated, necessitating periodic replacement of the batteries or at least recharging of the batteries. A third problem is that the use of the pod necessitates an inductive coupling system to transmit information from the externally-mounted unit back to the detectors or coils contained within the streamer. This presents reliability problems for the high speed data links which are utilized. The fourth problem, and a very severe one, is that the externally-mounted units generate acoustic noise since they are outside the streamer. Since they pose an impediment to the flow of water, they generate a certain amount of turbulence and bubbling such that with a large number of externally-mounted units, non-laminar flow conditions exist which, in some instances, can blanket the acoustic devices. A fifth reason that the pods are not exceptionally useful is that they are easily knocked off. For flotsam and jetsam such as a log in the water or the presence of a mooring cable, it will be appreciated that when the streamer slides by, the pods can be caught and knocked off which presents a severe cost problem.
A sixth and extremely important problem is that the pods are mounted of necessity to the skin of the streamer cable at one or two points. As is usual, the skin is not concentric with the axis of the streamer defined by the internally-carried strain members. The strain members are the most reliable indicator of the longitudinal axis of the streamer and are mounted through so-called "donuts" which are located just beneath the skin. Thus, the axis of a pod clamped to the skin does not necessarily correspond to the longitudinal axis of the streamer as defined by the strain members.
A seventh problem is the possibility that there are hydrodynamic instabilities in which the pod may cant or rack on its supports, thereby causing a misalignment between its axis and the axis of the strain members of the streamer. The externally-mounted pod, when canted, strums or jitters back and forth with respect to the axis of the streamer so that the heading readout from a pod-mounted compass is not a good indication of the heading of the streamer at the measuring point. The ability to determine heading accurately is of paramount importance in determining the accuracy of a survey.
An additional problem with respect to the externally-mounted pods is that while during calibration the mounts may accurately reflect the longitudinal axis of the streamer at one position immediately behind the boat, attaching the mounts at other positions along other streamers may result in a canting by as much as 1". While the clamping system can result in canting during calibration, it can also result in further canting when the pod is remounted down the streamer.
A subsidiary problem with the use of the externally-mounted pods is that the provision of any protuberances on a streamer does not lend itself easily to the streamer going through the shives or fair-leads and over the drum utilized for paying out the streamer.